1. Field of the Invention
This invention pertains in general to a radio-frequency sensor for reading data from passing tags and in particular for a low frequency sensor for reading passive tags.
2. Background Art
There is a general desire to know the location of goods in the supply chain. For example, a manufacturer may need real-time data regarding the location of needed parts. A common way to gather this real-time data is to record when the goods pass through supply-chain choke points, such as loading docks at airports, shipyards, train stations, and truck depots. A unique identifier is assigned to each container holding the goods (e.g., pallet, crate, truck, envelope) and the identifier, or data pertaining to the container or contents, is recorded when the container passes through the choke points.
One technique for assigning the unique identifier to shipping containers is placing a small electronic tag having the identifier on the container. A specialized reader senses the unique identifier as the container passes through the choke point.
In order for the reader to sense the identifier, the polarizations of the electromagnetic fields generated by the antennas in the reader and tag must be aligned. In most real-world implementations, however, the orientation of the tag, and, hence, the polarization of the field generated by the tag""s antenna, is unknown. For example, if the tag is attached to the top of a pallet being carried past the reader by a conveyor belt, the tag may be oriented parallel to the direction of travel, orthogonal to the direction of travel, or at some other angle. Therefore, there is a need in the art for a reader that can read tags regardless of the tags"" orientations.
The above need is met by a reader that uses antennas generating circularly polarized fields to read tags having any orientation. The reader includes two interleaved linear arrays of antennas, each antenna composed of a pair of crossed rods phased such that each crossed pair and thus each antenna generates a circularly polarized field. Fields of adjacent antennas in an array are arranged to have opposite rotation, thus, the vector components of the polarization in the direction across the width of the conveyor have peaks and nulls. To provide a more uniform field, the arrays are interleaved such that the nulls of one array""s fields are covered with the peaks of the other array""s fields. This arrangement allows the reader to read the identifier from the tag when the tag is at any orientation. A tag appearing at the extremity of the reader will be constrained by rails on the conveyor to be aligned in the direction of travel. The antennas at the extremity of the reader are polarized in the direction of travel in order to read such tags.